Physical Science

The right-hand rule is used to find the direction of the Current
magnetic field produced by a current.
Magnetic field
Is the direction of a wire’s magnetic field clockwise or Figure 2 You can use the right-hand
counterclockwise? Repeated measurements have suggested rule to find the direction of the
an easy way to predict the direction of a field. This method is magnetic field near a wire that is
summarized by the right-hand rule. carrying a current.

Right-hand If you hold a wire in your right hand and point your solenoid (SOH luh NOYD) a coil of wire
rule thumb in the direction of the positive current, the with an electric current in it
direction that your fingers curl is the direction of the
magnetic field.

The right-hand rule is illustrated in Figure 2. The right hand
grasps the wire while the thumb points in the direction of the
current. The fingers encircle the wire, and the fingertips point
in the direction of the magnetic field—counterclockwise when
viewed from the top. If the direction of the current were from
the top to the bottom of the page, the thumb would point
downward, and the magnetic field would run clockwise.
Remember: Never grasp or touch an uninsulated wire con-
nected to a power source. You could be electrocuted.

When using the right-hand rule, you should
point your thumb in which direction?

Solenoids and bar magnets have similar magnetic fields.
As you have learned, the magnetic field of a current-

carrying wire exerts a force on a compass needle. This force
causes the needle to turn in the direction of the wire’s mag-
netic field. However, this force is very weak for a small current.
Increasing the current in the wire is one way to increase the
force, but large currents can be fire hazards. Wrapping the
wire into a coil is a safer way to create a stronger magnetic
field, as Figure 3 shows. This device is called a solenoid.

In a solenoid, the magnetic field of each loop of wire adds
to the strength of the magnetic field of any neighboring loops.
The result is a strong magnetic field similar to the magnetic
field produced by a bar magnet. Like a magnet, a solenoid has
a north and south pole.

Figure 3 The magnetic field of
/ 4 a solenoid resembles the

magnetic field of a bar magnet.

$VSSFOU $VSSFOU

Section 2 M a g n e t i s mS ef rcot imo nE lXe cSt er icct iCounr rTei tnltes 627

Electromagnet 20 min

Procedure Analysis

1 Wind 1 m of insulated wire around 1. What type of device have you pro-
a large iron or steel nail. duced? Explain your answer.

2 Remove the insulation from the 2. What happened to the compass
ends of the wire. Hold the insulated needle after you reversed the direc-
wire so that the ends touch the ter- tion of the current? Explain your
minals of a 6 V lantern battery. observation?

3 Move a compass toward the nail. 3. After you detach the coil from the
cell, what can you do to make the
4 Flip the battery around so that the nail nonmagnetic?
current is reversed. Bring the com-
pass near the same part of the nail.

electromagnet (ee LEK troh MAG nit) a The strength of a solenoid can be increased.
coil that has a soft iron core and that acts as The strength of the magnetic field of a solenoid depends
a magnet when an electric current is in the
coil on the number of loops of wire and the amount of current in
the wire. In particular, more loops or more current can create
Academic Vocabulary a stronger magnetic field.

device (di VIES) a piece of equipment made The strength of a solenoid’s magnetic field can also be
for a specific use increased by inserting a rod made of a magnetic metal, such
as iron, through the center of the coils. The resulting device is
called an electromagnet. The magnetic field of the solenoid
causes the rod to become a magnet as well. Then, the mag-
netic field of the rod adds to the coil’s field and thus creates a
magnet that is stronger than the solenoid alone.

What relationship does the number of loops
of wire in a solenoid have to the strength of a solenoid?

Moving charges cause magnetism.
The movement of charges is the cause of all magnetism.

But what charges are moving in a bar magnet? Negatively
charged electrons moving around the nuclei of atoms make
magnetic fields. Atomic nuclei also have magnetic fields
because protons move within the nuclei. Each electron has a
property called electron spin, which also produces a tiny
magnetic field.

In most cases, the various sources of magnetic fields in an
element cancel out and leave the atom essentially nonmag-
netic. However, not all of the fields cancel in some materials,
such as iron, nickel, and cobalt. Thus, the magnetism of the
uncanceled fields in these materials combines to make the
materials magnetic overall.

628 CChhaapptteerr 1X8 C hMaapgt en re tTiist ml e

Why It Matters

Airport Security

Before traveling on an airplane, you are 1 Short pulses of current are passed through the coil at a
required to pass through a metal detector on
your way to the gate. You may also be asked rate of about 100 pulses per second. Each pulse creates a
to submit to a sweep with a hand-held metal short-lived magnetic field that quickly collapses.
detector. Metal detectors make use of
electromagnetism. Most airport metal $PJM
detectors use a system called pulse
induction (PI). These systems consist of a
power supply, a sensor circuit, and a coil of
metal through which a current can pass.

4FOTPS 1PXFS

.BHOFUJD mFME

$PJOT

2 When a metal object passes through the metal detector or

a hand-held detector passes near a metal object, an oppo-
site magnetic field is induced in the metal object.

3 When it detects

the opposing
magnetic field,
the sensor acti-
vates a light or
sound that sig-
nals the opera-
tor of the metal
detector.

UNDERSTANDING CONCEPTS
1. Does the field induced in the

metal object increase or
decrease the pulsed field?
WRITING IN SCIENCE
2. Choose another device used
in airport security, and
describe how this device
works.

629

Figure 4 A blow-dryer is one example Electromagnetic Devices
of an everyday device that uses an
electric motor. Many modern devices, such as blow-dryers and stereo
speakers, make use of the magnetic field produced by coils of
electric motor (ee LEK trik MOHT current-carrying wire. Devices such as the blow-dryer shown
uhr) a device that converts electrical in Figure 4, are able to function because the coils inside the
energy into mechanical energy devices work as motors.
galvanometer (GAL vuh NAHM uht
uhr) an instrument that detects, measures, Electric motors are machines that convert electrical
and determines the direction of a small energy into mechanical energy. V A motor can perform
electric current mechanical work when it is attached to an external device.
Electric motors are used in many devices around your home,
including many toys. Larger motors are found in washing
machines and clothes dryers, and simple motors can be found
in common household fans.

Galvanometers detect current.
Galvanometers are devices that are used to measure

current. The basic construction of a galvanometer is shown in
Figure 5. In all cases, a galvanometer detects current, or the
movement of charges in a circuit.

A galvanometer consists of a coil of insulated wire
wrapped around an iron core that can rotate between the
poles of a permanent magnet. When the galvanometer is
attached to a circuit, a current exists in the coil of wire. The
coil and iron core act as an electromagnet and produce a
magnetic field. This magnetic field interacts with the magnetic
field of the surrounding permanent magnet. The resulting
forces turn the core, which moves a needle along a scale.

A galvanometer can be used with other circuit elements to
function as an ammeter, which measures current, or as a
voltmeter, which measures voltage.

.PWBCMF DPJM

Figure 5 When there is

current in the coil of a

www.scilinks.org 4 / galvanometer, magnetic
Topic: Electromagnets repulsion between the coil
Code: HK80484
and the magnet causes the

coil to twist. How does the

direction of the motion

change if the current is

reversed?

4QSJOH

630 C h a p t e r 1 8 Magnetism

$PNNVUBUPS Figure 6 In an electric motor, the
/ current in the coil produces a magnetic
field that interacts with the magnetic
#SVTI field of the surrounding magnet and
thus causes the coil to turn.

4

#SVTI



#BUUFSZ

Motors use a commutator to spin in one direction. Cause and Effect
The arrow in Figure 6 shows how the coil of wire in a motor
Many factors influence how a
turns when a current is in the wire. Unlike the coil in a galva- motor works. As you read this
nometer, the coil in an electric motor keeps spinning. A device page and the previous page, look
called a commutator is used to make the current change for multiple causes and multiple
direction every time the flat coil makes a half revolution. This effects that enable a motor to turn.
commutator is two half rings of metal. Devices called brushes
connect the commutator to the wires from the battery.
Because of the slits in the commutator, charges must move
through the coil of wire to reach the opposite half of the ring.

So, the magnetic field of the coil changes direction as the
coil spins. In this way, the coil is repelled by both the north
and south poles of the magnet surrounding it. Because the
current keeps reversing, the loop rotates in one direction. If
the current did not keep changing direction, the loop would
simply bounce back and forth in the magnetic field until the
force of friction caused the loop to come to rest.

Section 2 Review

KEY IDEAS CRITICAL THINKING

1. Describe the shape of a magnetic field produced by 4. Predicting Outcomes Predict whether a solenoid
a straight wire that is carrying a current. suspended by a string could be used as a compass.

2. Determine the direction in which a compass needle 5. Analyzing Ideas A friend claims to have built a
will point when the compass is held above a wire motor by attaching a shaft to the core of a galva-
carrying positive charges that are moving west. nometer and removing the spring. Can this motor
rotate through a full rotation? Explain your answer.
3. Explain how galvanometers and electric motors
function.

Section 2 M a g n e t i s mS ef rcot imo nE lXe cSt er icct iCounr rTei tnltes 631

3 Electric Currents from
Magnetism

Key Ideas Key Terms Why It Matters

V What happens when a magnet is moved into or out electromagnetic Many common devices,
induction such as electric guitars
of a coil of wire? and speakers, rely on
generator electromagnetic induc-
V How are electricity and magnetism related? alternating tion to function.
V What are the basic components of a transformer?
current
transformer

Electric power plants convert mechanical energy—usually

the movement of water or steam—into electrical energy. How

can electrical energy be generated from mechanical energy?

Electromagnetic Induction

In 1831, Michael Faraday discovered that a current can be
produced by pushing a magnet through a coil of wire. This
happens without a battery or other source of voltage. V Moving
a magnet into and out of a coil of wire causes charges in
the wire to move. The process of creating a current in a circuit
by changing a magnetic field is called electromagnetic
induction. Electromagnetic induction is so fundamental that
it has become one of the laws of physics—Faraday’s law.

.BHOFUJD mFME Faraday’s An electric current can be produced in a circuit by
law changing the magnetic field crossing the circuit.

4 Consider the loop of wire moving between the two mag-
/ netic poles in Figure 1. As the loop moves into and out of the
magnetic field of the magnet, a current is induced in the
$VSSFOU %JSFDUJPO circuit. As long as the wire continues to move into or out of the
PG MPPQhT field in a direction that is not parallel to the field, an induced
NPUJPO current will exist in the circuit.

Figure 1 When the loop moves into Rotating the circuit or changing the strength of the mag-
or out of the magnetic field, a current is netic field will also induce a current in the circuit. In each
induced in the wire. What happens if case, a changing magnetic field is passing through the loop.
the loop is rotated between the You can use the concept of magnetic field lines to predict
magnets? whether a current will be induced. A current will be induced if
the number of field lines that pass through the loop changes.

632 CChhaapptteerr 1X8 C hMaapgt en re tTiist ml e

Electromagnetic induction obeys conservation of energy. Academic Vocabulary
Although electromagnetic induction may seem to create
violate (VIE uh LAYT) to fail to keep
energy from nothing, it does not. Electromagnetic induction
does not violate the law of conservation of energy. Pushing a electromagnetic induction (ee
loop through a magnetic field requires work. The greater the LEK troh mag NET ik in DUHK shuhn) the
magnetic field, the stronger the force required to push the process of creating a current in a circuit by
loop through the field. The energy required for this work changing a magnetic field
comes from an outside source, such as your muscles pushing
the loop through the magnetic field. So, electromagnetic
induction produces electrical energy, but energy is required
for electromangetic induction to occur.

The magnetic force acts on moving electric charges.
A charged particle moving in a magnetic field will experi-

ence a force in a direction that is at right angles to the direc-
tion of the magnetic field lines. This magnetic force is zero
when the charge moves in the same direction as the magnetic
field lines. The force is at its maximum value when the charge
moves perpendicularly to the field. As the angle between the
charge’s direction and the direction of the magnetic field
decreases, the force on the charge also decreases. This force
also acts on a wire that is carrying a current.

Can You Demonstrate 30 min
Electromagnetic Induction?

Procedure 7 Use two magnets, and hold them
alongside each other so that like
1 Connect each end of a hollow-core poles are touching.
wire coil, or solenoid, to a galva-
nometer, as shown in the photo. Analysis

2 Any current induced in the solenoid 1. What evidence indicates that a
will pass through the galvanometer. changing magnetic field induces a
For each of the following motions, current? What happens if you do not
record the direction in which the move the magnet at all?
galvanometer needle points and
the amount by which it moves. 2. Compare the current induced by a
south pole with the current induced
3 Insert the north pole of a bar mag- by a north pole.
net into the solenoid.
3. What two observations show that
4 Pull the magnet out of the more current is induced if the mag-
solenoid. netic field changes rapidly?

5 Turn the magnet around, and move 4. How does the amount of current
the south pole into and out of the induced depend on the strength of
solenoid. the magnetic field?

6 Vary the speed of your motion.

Section 3 E l e c t r i c C uSrercetni ot sn fXr o Sme cMt ai ognn eTtiitsl me 633

.BHOFUJD The magnetic force acts on wires carrying a current.
mFME When studying electromagnetic induction, you may find it

helpful to imagine the individual charges in a wire. Imagine
that the wire in a circuit is a tube full of charges, as illustrated
in Figure 2. When the wire is moving perpendicularly to a
.PUJPO magnetic field, the force on the charges is at a maximum. In
PG XJSF this case, a current is in the wire and circuit. When the wire is
moving parallel to the field, no current is induced in the wire.
When the wire in a circuit moves Because the charges are moving parallel to the field, they
perpendicularly to the magnetic experience no magnetic force.
field, the current induced in the
Generators convert mechanical energy into electrical
wire is at a maximum. energy.

.BHOFUJD .PUJPO Generators are similar to motors but convert mechanical
mFME PG XJSF energy into electrical energy. If you expend energy to do work
on a simple generator, such as the one in Figure 3, the loop of
wire inside turns within a magnetic field and thus produces a
current. For each half rotation of the loop, the current pro-
When the wire moves parallel to duced by the generator reverses direction. A current that
the magnetic field, no current is changes direction at regular intervals is called an alternating
current (AC).
induced in the wire.
The generators that produce the electrical energy that you
Figure 2 A current is induced in a use at home are alternating-current generators. The current
closed circuit when the circuit supplied by the outlets in your home and in most of the world
moves through a magnetic field. is alternating current. The glowing light bulb in Figure 3 indi-
cates that the coil turning in the magnetic field of the magnet
Keyword: HK8MAGF2 creates a current. The magnitude and direction of the current
that results from the coil’s rotation vary depending on the
orientation of the loop in the field.

What must be done to produce a current
using a generator?

4MJQ SJOHT Figure 3 In an
alternating-current
generator (JEN uhr AYT uhr) a machine / generator, the mechanical
that converts mechanical energy into energy of the loop’s
electrical energy 4 rotation is converted into
electrical energy when a
alternating current (AWL tuhr NAYT current is induced in the
ing KUHR uhnt) an electric current that wire. The current lights
changes direction at regular intervals the light bulb.
(abbreviation, AC)
#SVTI

#SVTI

634 CChhaapptteerr 1X8 C hMaapgt ne re tTiist ml e

Why It Matters Magnetic guitar pickups contain a permanent
magnet that is wrapped with a coil of wire. The pole
How Do Electric of the magnet is directly under the guitar strings.
Guitars Work? The guitar strings are made of a magnetic material—
usually steel, nickel, or both. As a string vibrates, it
The word pickup refers to a device induces a current in the pickup coil, and the
that “picks up” the sound of an vibration is converted into an electrical signal.
instrument and turns that sound into
an electrical signal. The most common M.BaHgOnFeUTts St4riUnSJOgHT
type of electric guitar pickup uses Wire c8oJiSlFs DPJM
electromagnetic induction to convert
string vibrations into electrical energy. 7JCSBUJVOiHb TraUStJOinHg
Conversely, a speaker converts an string
electrical signal back into sound.
&UMPF EDBUlNSetJoDQcBtMJarMm imTFcJSaHpOl lBsifMiigenr al
Electric guitar pickups come in
many styles, and a single electric
guitar often has two or three
kinds of pickups. One kind of
pickup, the humbucker, is
designed to reduce the noise, or
hum, that simpler pickups make
because of alternating current.

Speakers contain a permanent magnet and a coil of
wire attached to a flexible paper cone. Current in
the coil induces a magnetic field, which causes the
paper cone to move. Varying the current changes
how much and how fast the cone vibrates. These
vibrations produce sound waves.

4 1BQFS
/ DPOF

4
7PJDF
DPJM

www.scilinks.org UNDERSTANDING CONCEPTS
Topic: Electromagnetic 1. How are vibrations in a paper

Induction cone related to sound?
Code: HK80481 CRITICAL THINKING
2. Explain why a magnetic pickup

does not work with nylon
guitar strings.

635

Figure 4 Induced Current in a Generator

Position of loop Amount of current Graph of current versus angle of rotation

.BHOFUJD mFME

zero current $VSSFOU 3PUBUJPO
     BOHMF

.BHOFUJD mFME $VSSFOU 3PUBUJPO
     BOHMF
maximum current

.BHOFUJD mFME

zero current $VSSFOU 3PUBUJPO
     BOHMF

.BHOFUJD mFME maximum current $VSSFOU 3PUBUJPO
(opposite direction) BOHMF
    

.BHOFUJD mFME

zero current $VSSFOU 3PUBUJPO
BOHMF
    

Cause and Effect The amount of current produced by an AC generator
changes with time.
As you read this page, look for
cause-and-effect markers. Make a Study the diagrams in Figure 4. When the loop is perpen-
two-column table or appropriate dicular to the field, the current is zero. Recall that a charge
FoldNote, and label the columns moving parallel to a magnetic field experiences no magnetic
“Cause” and “Effect.” Fill the table force. This is the case here. The charges in the wire experience
with information that you learn no magnetic force, so no current is induced in the wire.
about induced current.
As the loop turns, the current increases until it reaches a
maximum. When the loop is parallel to the field, charges on
either side of the wire move perpendicularly to the magnetic
field. Thus, the charges experience the maximum magnetic
force, and the current is large. Current decreases as the loop
rotates. When the loop is perpendicular to the magnetic field
again, the current once again reaches zero. As the loop contin-
ues to rotate, the direction of the current reverses.

636 C h a p t e r 1 8 Magnetism

Generators produce the electrical energy that you use in Integrating Biology
your home.
Biomagnets Many types of bacteria
Large power plants use generators to convert mechanical contain magnetic particles of iron
energy into electrical energy. The mechanical energy used in a oxide and iron sulfide. Encased in a
commercial power plant comes from a variety of sources. One membrane within the cell, these
of the most common sources is running water. Dams are built particles form a magnetosome. The
to harness the kinetic energy of falling water. Water is forced magnetosomes in a bacterium spread
through small channels at the top of a dam. As the water falls out in a line and align with Earth’s
to the base of the dam, it turns the blades of large turbines. magnetic field. As the cell uses its
The turbines are attached to a core wrapped with many loops flagella to swim, it travels along a
of wire that rotate within a strong magnetic field. The end north-south axis. Recently, magnetite
result is electrical energy. crystals have been found in human
brain cells, but the role that these
Coal power plants use the energy from burning coal to particles play remains uncertain.
make steam that eventually turns the blades of turbines. Other
sources of energy are nuclear power (fission), wind power, www.scilinks.org
geothermal power, and solar power. Topic: Generators
Code: HK80643
Some mechanical energy is always lost as waste heat, and
resistance in the wires of the generator reduces the electrical
energy that is available. Many power plants are not very ef-
ficient. Methods of producing energy that are more efficient
and safer are constantly being sought.

What are three sources of mechanical energy
used by power plants to produce electrical energy?

The Electromagnetic Force Figure 5 An electromagnetic wave
consists of electric and magnetic field
So far, you have learned that moving charges produce waves that are at right angles.
magnetic fields and that changing magnetic fields cause
electric charges to move. V Electricity and magnetism are 0TDJMMBUJOH NBHOFUJD mFME
two aspects of a single force, the electromagnetic force.
0TDJMMBUJOH FMFDUSJD mFME
The energy that results from the electromagnetic force is %JSFDUJPO PG UIF
electromagnetic energy. Light is a form of electromagnetic FMFDUSPNBHOFUJD XBWF
energy. Visible light travels as electromagnetic waves, or EM
waves, as do other forms of radiation, such as radio signals
and X rays. As Figure 5 shows, EM waves are made up of oscil-
lating electric and magnetic fields that are perpendicular to
each other. This is true of any type of EM wave regardless of
the frequency.

Both the electric and magnetic fields in an EM wave are
perpendicular to the direction in which the wave travels. So,
EM waves are transverse waves. As an EM wave moves along,
the changing electric field generates the magnetic field. The
changing magnetic field generates the electric field. Because
each field regenerates the other, EM waves are able to travel
through empty space.

Section 3 E l e c t r i c C u r r e n t s f r o m M a g n e t i s m 637

Primary circuit Secondary circuit Transformers

Figure 6 A transformer uses the You may have seen metal cylinders on power line poles in
alternating current in the primary circuit your neighborhood. These cylinders hold devices called
to induce an alternating current in the transformers, devices that increase or decrease the voltage of
secondary circuit. alternating current. V In its simplest form, a transformer
consists of two coils of wire wrapped around opposite sides
transformer (trans FAWRM uhr) a of a closed iron loop. In the transformer shown in Figure 6,
device that increases or decreases the one wire is attached to a source of alternating current, such as
voltage of alternating current a power outlet, and is called the primary circuit. The other
wire is attached to an appliance, such as a lamp, and is called
the secondary circuit.

When there is current in the primary circuit, this current
creates a changing magnetic field in the primary coil that mag-
netizes the iron core. The changing magnetic field of the iron
core then induces a current in the secondary coil. The direc-
tion of the current in the secondary coil changes every time
the direction of the current in the primary coil changes.

Transformers can increase or decrease voltage.
The voltage induced in the secondary circuit of a trans-

former depends on the number of loops, or turns, in the coil,
as shown in Figure 7. In step-up transformers, the primary coil
has fewer turns than the secondary coil does. In this case, the
voltage across the secondary coil is greater than the voltage
across the primary coil. In step-down transformers, the sec-
ondary coil has fewer loops than the primary coil does. The
voltage across the secondary circuit is lower than the voltage
across the primary circuit.

Figure 7 How Transformers Change Voltage

1SJNBSZ DPJM 4FDPOEBSZ DPJM 1SJNBSZ DPJM 4FDPOEBSZ DPJM

In a step-up transformer, the primary coil has In a step-down transformer, the primary coil has
fewer loops than the secondary coil does. The more loops than the secondary coil does. The
voltage in the secondary coil must be higher than
voltage in the secondary coil must be lower than
the voltage in the primary coil. the voltage in the primary coil.

638 CChhaapptteerr 1X8 C hMaapgt en re tTiist ml e

Transformers must obey the law of conservation of Figure 8 Step-down transformers like
energy. the ones shown here are used to
reduce the voltage across power lines.
Transformers may seem to provide something—more What is the advantage of using a
voltage—for nothing. But they do not. The power output of the lower voltage within homes?
secondary coil is, at best, equal to the power input to the
primary coil. One cannot get more electrical energy per unit of
time, or power, out of the transformer than one puts into it.
For this reason, the current in the secondary coil of a step-up
transformer is always less than the current in the primary coil.

Real transformers are not perfectly efficient. Some of the
energy that is put into a transformer is lost as heat because of
resistance in the coils. The power lost increases quickly as
current increases. To decrease loss and maximize the energy
that is delivered, power companies use a high voltage and a
low current when transferring power over long distances.

Transformers are used in the transfer of electrical
energy.

Step-up and step-down transformers are used in the
transmission of electrical energy from power plants to homes
and businesses. A step-up transformer is used at or near a
power plant to increase the voltage to about 120,000 V. This
high voltage limits the loss of energy that the resistance of the
transmission wires causes. Then, step-down transformers like
the ones in Figure 8 are used near homes to reduce the voltage
to about 120 V. This low voltage is much safer to use in homes.

Section 3 Review

KEY IDEAS CRITICAL THINKING

1. Identify which of the following will not increase the 5. Making Predictions For each of the following
current induced in a wire loop moving through a actions, predict the movement of the needle of a
magnetic field. galvanometer attached to a coil of wire. Assume that
a. increasing the strength of the magnetic field the north pole of a bar magnet has been inserted
b. increasing the speed of the wire into the coil, which causes the needle to deflect to
c. rotating the loop until it is perpendicular to the right.
the field a. pulling the magnet out of the coil
b. letting the magnet rest in the coil
2. Explain how hydroelectric power plants use moving c. thrusting the south pole of the magnet into
water to produce electricity. the coil

3. Explain how electricity and magnetism are related 6. Determining Cause and Effect A spacecraft orbit-
to one another. ing Earth contains a coil of wire. An astronaut
measures a small current in the coil even though
4. Determine whether the following statement the coil is not connected to a battery and the space-
describes a step-up transformer or a step-down craft does not contain any magnets. What is causing
transformer: The primary coil has 7,000 turns, and the current?
the secondary coil has 500 turns.

Section 3 E l e c t r i c C uSrercetni ot sn fXr o Sme cMt ai ognn eTtiitsl me 639

50 min

What You’ll Do Making a Better
Electromagnet
V Build several electromagnets.
V Determine how many paper clips In a Quick Lab earlier in this chapter, you made an electromagnet by
using batteries and a wire coil. In this lab, you will experiment with the
each electromagnet can lift. characteristics that make an electromagnet stronger.
V Analyze your results to identify the
Asking a Question
features of a strong electromagnet.
What combination of various batteries, wires, and metal rods will make
What You’ll Need the strongest electromagnet?

batteries, D-cell (2) Building an Electromagnet
battery holders (2) 1 Review the basic steps in making an electromagnet by looking at the
electrical tape
metal rods (1 iron, 1 tin, 1 aluminum, Quick Lab in Section 2.

and 1 nickel) 2 On a blank sheet of paper, prepare a data table like the one shown
paper clips, small (1 box) in this activity.
wire, extra-insulated
wire, insulated, thick, 1 m long 3 Wind the thin wire around the thickest metal core. Carefully pull the
wire, insulated, thin, 1 m long core out of the center of the thin wire coil. Using the thick wire,
wire stripper repeat the steps above. You now have two wire coils that can be
used to make electromagnets. CAUTION: Handle the wires only
Safety where they are insulated.

Forming and Testing a Hypothesis

4 Think about the following, and predict the features that the
strongest electromagnet would have.
a. Which metal rod would make the best core?
b. Which of the two wires would make a stronger electromagnet?
c. How many coils should the electromagnet have?
d. Should the batteries be connected in series or in parallel?

640 CChhaappt teer r X1 8 Magnetism

Sample Data Table: Differences in Electromagnets

Electromagnet Wire (thick No. Core (iron, tin, Batteries (series No. of
number or thin) of alum., or nickel) or parallel) paper
coils clips lifted

1

2

3

4

5

6

Designing Your Experiment

5 With your lab partners, decide how you will determine the features
that combine to make a strong electromagnet.

6 In your lab report, list each step you will perform in your experiment.

Performing Your Experiment

7 After your teacher approves your plan, carry out your experiment.
You should test all four metal rods, both thicknesses of wire, and
both battery connections (series and parallel). Count the number of
coils of wire in each electromagnet that you build.

8 Record your results in your data table.

Analysis

1. Explaining Events Which wire made a stronger electromagnet: the
thick wire or the thin wire? How can you explain this result?

2. Explaining Events Which metal cores made the strongest
electromagnets? Why?

3. Explaining Events Could your electromagnet pick up more paper
clips when the batteries were connected in series or when they
were connected in parallel? Explain why.

Communicating Your Results

4. Drawing Conclusions What combination of wire, metal core, and
battery connection made the strongest electromagnet?

Extension

Suppose someone tells you that your
conclusion is invalid because each time you
tested a magnet on the paper clips, the
paper clips became more and more
magnetized. How could you show that your
conclusion is valid?

CChhaapptteerr TLiat bl e 641

Making Predictions Science Skills

One of the main goals of science is to explain the nature of the world Technology
around us. Another important goal is to allow us to predict—with a Math
reasonable degree of confidence and accuracy—what will happen in Scientific Methods
the future. Predictions based on hypotheses help scientists design Graphing
experiments. Predictions based on established scientific laws or
theories help scientists apply science to solve real-world problems.

1 Predictions Based on Hypotheses • Observations: Magnets stick to a metal
refrigerator door, but not to a door
• A hypothesis is formed from observations that you have made of wood or plastic.
made or data that you have collected.
• Hypothesis: Magnets are attracted to
• After you have a hypothesis, you should make two any kind of metal, but not to anything
kinds of predictions: one that states what will happen else.
if the hypothesis is true and one that states what will
happen if the hypothesis is not true. • Predictions if hypothesis is true: (1) I
will not be able to find a metal to which
• These predictions can help you plan how to test the a magnet will not stick and (2) I will not
hypothesis. be able to find a nonmetal to which a
magnet will stick.

• Predictions if hypothesis is NOT true:
(1) I will be able to find a metal to which
a magnet will not stick and (2) I will
be able to find a nonmetal to which a
magnet will stick.

2 Predictions Based on Established Theories • Established scientific law: Observa-
tions of sunspots (dark patches on the
• After a hypothesis has been tested and confirmed surface of the sun) have shown that
repeatedly by many scientists, the hypothesis can be the spots grow and fade in an 11-year
accepted as a scientific law or as part of a theory. You cycle.
can then use this theory to make further predictions.
• Fact: The last time the sunspot cycle
• You can use equations to make precise, quantitative was at a maximum was in 2001.
predictions.
• Prediction: The next time the cycle will
• You can extrapolate, or continue the trend suggested be at a maximum will be around 2012.
by past known data, using graphs to obtain quantitative
predictions.

1. Suppose that you want to test the hypothesis 2. The “sunspot number” is an index that is used
that Earth’s North Pole is like the south pole of a to measure the amount of sunspot activity on
bar magnet. Predict how a compass will behave a given day. Use the Internet to research the
around a bar magnet if (a) the hypothesis is true sunspot cycle for the sun. Predict what the peak
and (b) the hypothesis is not true. sunspot number will be in 2023.

642 C h a p t e r 1 8 Magnetism

18 KEYWORD: HK8MAGS

Key Ideas Key Terms

Section 1 Magnets and Magnetic Fields magnetic pole, p. 619
magnetic field, p. 621
V Magnets All magnets have two poles that cannot
be isolated. Like poles repel each other, and unlike
poles attract each other. (p. 619)

V Magnetic Fields Magnets repel or attract each
other because of the interaction of their magnetic
fields. (p. 621)

V Earth’s Magnetic Field Earth’s magnetic field lines
run from geographic south to geographic north. The
magnetic north pole is in Antarctica, and the
magnetic south pole is in northern Canada. (p. 623)

Section 2 Magnetism from Electric Currents solenoid, p. 627
electromagnet, p. 628
V Electromagnetism When a wire carries a strong, electric motor, p. 630
steady current, the needles of any compasses nearby galvanometer, p. 630
move to align with the magnetic field created by the
electric current. (p. 626)

V Electromagnetic Devices A motor can perform
mechanical work when it is attached to an external
device. Electric motors convert electrical energy into
mechanical energy. (p. 630)

Section 3 Electric Currents from Magnetism electromagnetic
induction, p. 632
V Electromagnetic Induction Moving a magnet into
and out of a coil of wire causes charges in the wire generator, p. 634
to move. A current is produced in a circuit by a
changing magnetic field. (p. 632) alternating current,

V The Electromagnetic Force Electricity and p. 634
magnetism are two aspects of a single force, the
electromagnetic force. Electromagnetic waves consist transformer, p. 638
of magnetic and electric fields oscillating at right
angles to each other. (p. 637)

V Transformers A transformer consists of two coils
or wire wrapped around opposite sides of a closed
iron loop. In a transformer, the magnetic field
produced by a primary coil induces a current in a
secondary coil. (p. 638)

M a g n e t i s m 643

18

1. Suffixes The magnetometer was first designed 9. A compass held directly below a current-carrying
in 1833 by Carl Friedrich Gauss, a German
mathematician and scientist. A magnetometer wire in which positive charges are moving north
is also sometimes called a gaussmeter. A gauss
is also a unit for measuring the strength of will point
magnetic fields. Given this information, what do
you think a magnetometer is? a. $VSSFOU / c. $VSSFOU /

USING KEY TERMS 8& 8&

2. Use the terms magnetic pole and magnetic field 44
to explain why the north pole of a compass
needle points toward northern Canada. b. $VSSFOU / d. $VSSFOU /

3. Write a paragraph explaining the advantages and 8& 8&
disadvantages of using a magnetic compass to
determine direction. Use the terms magnetic 4 4
pole and magnetic field in your answer.
10. An electric motor uses an electromagnet to
4. How does a galvanometer measure electric change
current? How is it similar to and different from a. mechanical energy into electrical energy.
an electric motor? b. magnetic fields in the motor.
c. magnetic poles in the motor.
5. What is the purpose of a commutator in an d. electrical energy into mechanical energy.
electric motor?
11. An electric generator is a device that can convert
6. Use the terms generator and electromagnetic a. nuclear energy into electrical energy.
induction to explain how kinetic energy of falling b. wind energy into electrical energy.
water is used to generate electrical energy. c. energy from burning coal into electrical
energy.
U N DERSTAN DI NG KEY I DEAS d. All of the above

7. If the poles of two magnets repel each other, 12. The process of producing an electric current by
a. both poles must be south poles. moving a magnet into and out of a coil of wire
b. both poles must be north poles. is called
c. one pole is south and the other is north. a. magnetic deduction.
d. the poles are the same type. b. electromagnetic induction.
c. magnetic reduction.
8. The part of a magnet where the magnetic field d. electromagnetic production.
and forces are strongest is called a magnetic
a. field. 13. In a transformer, the voltage of a current will
b. pole. increase if the secondary circuit
c. attraction. a. has more turns than the primary circuit does.
d. repulsion. b. has fewer turns than the primary circuit does.
c. has the same number of turns that the pri-
mary circuit does.
d. is parallel to the primary circuit.

644 CChhaappt teer r X1 8 Magnetism

EXPLAINING KEY IDEAS 20. Applying Technology Use your imagination
and your knowledge of electromagnetism to
14. How could you use a compass that has a invent a useful electromagnetic device. Use a
magnetized needle to determine if a steel nail is computer-drawing program to make sketches of
magnetized? your invention, and write a description of how it
works.
15. What happens to the magnetic domains in a
material when the material is placed in a strong 21. Applying Knowledge What do adaptors do to
magnetic field? voltage and current? Examine the input/output
information on several electrical adapters to
INTERPRETING GRAPHICS The diagram below find out. Do they contain step-up or step-down
shows a wire wrapped around a magnetic compass. transformers?
Use the diagram to answer question 16.
22. Relating Concepts Research how electro-
magnetism is used in containing nuclear fusion
reactions. Write a report on your findings.

16. Which of the following might be the purpose of 23. Induced Current The figure below is a graph ofCurrent
the device shown here? current versus rotation angle for the output of an
a. to measure the amount of voltage across alternating-current generator.
the wire a. At what point(s) does the generator produce
b. to determine the direction of the current in no current?
the wire b. Is the current produced at point B less than or
c. to find the resistance of the wire more than the current at points C and E?
c. Is the current produced at point D less than
17. Transformers are usually used to raise or lower or more than the current at points C and E?
the voltage across an alternating-current circuit. d. What does the negative value for the current
Can a transformer be used in a direct-current at point D signify?
circuit? Can a transformer be used if the direct
current is pulsating (turning on and off)? B

CRITICAL THINKING A
CE
18. Understanding Systems Fire doors are doors Rotation
that, when closed, can slow the spread of fire angle
from room to room. In some buildings, fire
doors are held open by electromagnets. Explain 0º 90º 180º 270º 360º
why electromagnets rather than permanent
magnets are used. D

19. Making Decisions You have two iron bars and
a ball of string. One bar is magnetized, and the
other is not magnetized. How can you determine
which bar is magnetized?

C h aCpht ae pr tReer vTi ei twl e 645

Understanding Concepts Reading Skills

Directions (1–4): For each question, write on a Directions (7–10): Read the passage below.
sheet of paper the letter of the correct answer. Then, answer the questions that follow.

1. A straight vertical wire is carrying an electric GETTING IT ON TAPE
Magnetic tape consists of a thin plastic
current. Positive charges are flowing straight strip bonded to a coating of ferric oxide
powder. The ferric oxide, Fe2O3, makes the
down. What is the direction of the magnetic tape magnetizable. Early tape recorders were
first developed in Germany and Britain.
field generated by the wire as viewed from The first tape recorder used by the British
Broadcasting Corporation in 1932 was a huge
above? machine. It used steel razor tape that was
3 mm wide and 0.08 mm thick. The tape
A. straight up C. clockwise had to be run at 90 m/min, so the length of
tape required for a half-hour program was
B. straight down D. counterclockwise nearly 3 km long, and a full reel had a mass of
25 kg. Furthermore, the quality of the sound
2. What type of device is used to measure the experienced considerable degradation in the
current in an electromagnet? recording and playback process.
F. an electric motor Higher-quality sound recording was
G. a galvanometer developed in Germany during the late 1930s.
H. a generator During World War II, the Allies became aware
I. a solenoid of German radio broadcasts that seemed to
be recorded. However, the audio quality and
3. A charged particle is moving through a duration of the recordings were far greater
magnetic field. In which direction is the than Allied technology would allow. At the
particle moving when the magnetic force end of the war, the Allies captured a number
acting on the particle is at its greatest? of German Magnetophon recorders from
A. in the same direction as the magnetic Radio Luxembourg, and commercial-quality
field lines magnetic recording entered the English-
B. in the opposite direction from the mag- speaking world.
netic field lines
C. at right angles to the magnetic field lines 7. How is a tape coated with Fe2O3 similar to a
D. clockwise around the magnetic field lines steel wire?

4. In an AC generator, a loop of wire rotates 8. Was the sound of a British recording during
World War II better or worse after the
between two magnetic poles. At what angle(s) recording was played?

of rotation relative to the magnet does the loop 9. The Allies captured recording devices called
magnetophons. Why is this name appropriate?
generate the most current?
10. Why would bringing audiotape near a powerful
F. 0˚ and 180˚ H. 180˚ magnet be a bad idea?

G. 90˚ and 270˚ I. 360˚

Directions (5–6): For each question, write a short
response.

5. How would a compass located precisely at the
Earth’s geographic north pole behave?

6. What is the difference between a solenoid and
an electromagnet?

646 CChha appt et er rX X1 8 C Mh aapgtneer t iTsimt l e

Interpreting Graphics

The diagram below shows an electric circuit that includes a solenoid.
Use this diagram to answer questions 11–12.

ELECTRIC CIRCUIT WITH SOLENOID

"
#

%

$
4PVSDF

9 &
'

11. What is the direction of the magnetic field at point X due to the

current in section EF?

A. into the page C. to the left

B. out of the page D. to the right

12. To which point is the north pole of the solenoid the closest?

F. A H. C

G. B I. D

The following graphic shows four bar magnets and the magnetic fields
that they generate. Use this graphic to answer questions 13–14.

MAGNETIC FIELD OF TWO PAIRS OF BAR MAGNETS

"#$%

& ' () When using an illustration
that has labels to answer
13. Suppose that A and E are the north poles of their magnets. What other a question, read the
labels carefully, and then
points have a north polarity? check that the answer
you choose matches
A. B and G C. D and G your interpretation of the
labels.

B. C and H D. D and H

14. Which two magnets could combine their magnetic fields into one long
magnet without being rotated? What would happen to their poles?

SS tt aa nn dd aa rr dd ii zz eeCddh aTTepe stsett rPPTrr eei tppl e 647



Earth and
Space Science

Chapter 19 The Solar System .............................. 650
Chapter 20 The Universe ........................................ 690
Chapter 21 Planet Earth .......................................... 726
Chapter 22 The Atmosphere ............................... 770
Chapter 23 Using Natural Resources ........... 804

649

19 The Solar System

Chapter Outline

1 Sun, Earth,
and Moon

The View from Earth
A Family of Planets
The Moon

2 The Inner and
Outer Planets

The Inner Planets
The Gas Giants
Beyond the Gas Giants

3 Formation of the
Solar System

Early Astronomy
The Nebular Hypothesis
Rocks in Space
How the Moon Formed
Do Other Stars Have Planets?

650

Why It Matters

The Mars Expedition Rovers Spirit
and Opportunity (shown here)
were meant to study Mars for about
90 days but sent back information
for several years. Even the scientists
who built the Rovers were
surprised. In this chapter, you will
learn more about our solar system
and how scientists study it.

20 min

Observing Stars

Go outside on a clear evening at dusk, and look up
at the sky. Spend 20 min counting stars as they
become visible.

Questions to Get You Started
1. How many stars did you count in 20 min?
2. Do you see any lights in the sky that might not

be stars? What do you think they might be, and
how would you find out what they are?

651

These reading tools can help you learn the material in this
chapter. For more information on how to use these and other
tools, see Appendix A.

Word Origins Mnemonics

Names of the Planets The ancient Romans Order of the Planets A mnemonic is a

named the five planets that they could see moving sentence or phrase that you can create to help
in the night sky after gods from Roman mythology: you remember specific information. For example,
Mercury, Venus, Mars, Jupiter, and Saturn. When the order of the planets from the sun outward
Uranus and Neptune were discovered, they were is Mercury, Venus, Earth, Mars, Jupiter, Saturn,
also named for Roman gods. Uranus, and Neptune. You can use a
mnemonic to help you remember this order.
Your Turn Each planet was named for the god in
mythology that has something in common with the My Very Empathetic Mother Just
planet. As you learn about the planets, complete Served Us Nachos.
the list of connections between the planets and the
Roman gods for which they are named. Notice that the first letter of each word is the
same as the first letter of the corresponding planet
PLANET CHARACTERISTIC CHARACTERISTIC in the order.
Mercury
OF PLANET OF ROMAN GOD Your Turn Practice saying this mnemonic to help
you learn and remember the order of the planets.
fastest-moving fast messenger of Make up a new mnemonic that you could use to
planet the gods remember the order of the planets.

Jupiter largest planet king of the Roman
gods

Graphic Organizers The solar A flat,
nebula rotating disk
Chain-of-Events Chart Use a
collapsed. formed
chain-of-events chart when you need to
remember the steps of a process.

Your Turn As you read Section 3, com-
plete the chain-of-events chart that out-
lines the formation of the solar system.

1 The first step of the process is written
in the first box.

2 The next step of the process is written
in the second box, and an arrow
shows the order of the process.

3 Continue adding boxes and arrows
until the process for the formation of
the solar system is complete.

652 C h a p t e r 1 9 The Solar System

1 Sun, Earth, and Moon

Key Ideas Key Terms Why It Matters

V Why does the night sky look the way it does from planet Studying the solar
solar system system and its various
Earth? satellite planets can help us
phase understand our own
V What objects make up the solar system? eclipse planet better.
V How does the moon affect Earth?

The sun, moon, and stars appear to rise and set each day

because Earth spins on its axis. The stars that are visible at

night change throughout the year as Earth orbits the sun.

These two motions affect our view of the sky.

The View from Earth planet (PLAN it) a celestial body that orbits
the sun, is round because of its own gravity,
If you go out and look at the sky tonight and then look at it and has cleared the neighborhood around its
again in six months, it will look different. In fact, you can see orbital path
some star groups move across the sky in just a few hours!
V The positions of objects in the sky change over time Figure 1 The sun looks small to us
because Earth, and everything else in the universe, is only because it is so far away. If it were
constantly moving. Earth’s spinning on its axis and orbiting next to Earth, the sun would look
the sun create apparent movement in the sky. Many distant many times larger.
objects, such as stars, move slowly and appear to move to-
gether. Closer objects, such as planets and comets, move &BSUI
across this background of stars. A planet is a celestial body
that orbits the sun, is round because of its own gravity, and art on white background, sun
has cleared the area around its orbital path. and earth in size comparison

The sun is the closest star to Earth. 4VO
It took thousands of years for people to realize that the sun
653
is a star. The sun is the brightest object in the sky because it is
so close to us. Our atmosphere scatters the sun’s light and
makes the daytime sky so bright that we cannot see the other
stars. But the sun is an average star. It is not particularly hot or
cool and is of average size. Its diameter is 1.4 million kilo-
meters. As Figure 1 shows, the sun’s diameter is much
larger than Earth’s. The sun has a mass that is about
330,000 times the mass of Earth.

Figure 2 These tulips open to the Nature uses the sun to set daily cycles.
sun’s rays during the day and close at The sun is the major external source of heat and light for
night. What are some other plants
or animals that behave differently Earth. As Earth turns on its axis every 24 h, we see the sun rise
during the day and night? and set. Many patterns of animal and human life, such as
rising in the morning, eating meals at certain times, and
sleeping at night, follow the sun’s cycle.

As each year progresses, you can watch the growing sea-
sons of plants change. Some plants, such as the tulips shown
in Figure 2, are very sensitive to sunlight. They open during the
day and close at night.

Energy from the sun is also a main cause of weather pat-
terns and ocean currents on Earth.

Name two ways in which the sun affects life
on Earth. (See Appendix E for answers to Reading Checks.)

Planets and distant stars are visible in the night sky.
When we pick out the shapes of constellations, or groups of

stars organized in recognizable patterns, we use some of the
same patterns that ancient Greek scholars saw and named.
These scholars also watched the five bright planets wander in
regular paths among the stars. Figure 3 shows Saturn wander-
ing slowly through the constellation Leo. By watching the sky
for many years, the ancient Greeks calculated that the stars
were more distant than the planets were. More than a thou-
sand years later, after the invention of the telescope, people
found other objects in the night sky, including many faint
stars; two more planets, Uranus and Neptune; and several
other large celestial bodies, such as Ceres and Pluto.

October August
2007 2007
September
2007

Saturn

Figure 3 The planet Saturn
moves against the background
of stars in the constellation Leo,
named for its lionlike shape.

654 C h a p t e r 1 9 The Solar System

A Family of Planets solar system (SOH luhr SIS tuhm) the
sun and all of the planets and other bodies
A football game consists of a ball, players, referees, and that travel around it
coaches, which all interact according to a set of rules. The
solar system also has its objects and its own set of rules. V The Forward
solar system is the sun and all of the objects that orbit it. motion
The sun is the most important part of our system and makes
up nearly 99% of the total mass of the solar system. The eight Earth
planets and their moons make up most of the remaining 1%.
The solar system also contains many other smaller objects, Pull of
such as dwarf planets, asteroids, comets, dust, and gas. gravity

Gravity holds the solar system together. Resulting path
Every object in the solar system pulls on every other ob- (orbit)
Sun
ject. The force of gravity between two objects depends upon
their masses and the distance between them. The greater the Figure 4 The pull of gravity causes
mass is, the larger the gravitational force an object exerts on Earth to fall toward the sun and
another object. The closer two objects are to each other, the changes what would be a straight line
stronger the gravitational pull is between them. The sun exerts into a curve.
the largest force in the solar system because its mass is so
large. The pull of the sun keeps Earth in its orbit, as shown in
Figure 4. Imagine swinging a ball on a string. If you let go of the
string, the ball will fly off in a straight line. Without the sun’s
pull, Earth and all the other planets would similarly shoot off
into space. Figure 5 shows the orbits of the planets in order of
distance from the sun. Even at great distances, the sun keeps
the planets in their orbits.

Gravity is also the force that keeps moons orbiting around
planets. You experience gravity as the force that keeps you on
Earth. Even though Jupiter is far more massive than Earth, you
do not notice its pull on you because it is too far away.

/FQUVOF

4VO 4BUVSO
&BSUI

+VQJUFS

6SBOVT

Figure 5 The sun’s gravita-
tional force holds the planets
in almost circular orbits.

Section 1 S u n , E a r t h , a n d M o o n 655

+VQJUFS 4BUVSO

.FSDVSZ 6SBOVT /FQUVOF
&BSUI

7FOVT .BST

Figure 6 The planets in the solar Eight planets orbit the sun.
system are shown in relative scale. The Planets can be seen because their surfaces or atmospheres
sun’s diameter is almost 10 times
Jupiter’s. The distances between the reflect sunlight. A planet’s distance from the sun determines
planets are not shown to scale. how long it takes to orbit the sun. Mercury, the closest, takes
only 88 days to orbit the sun. Earth takes one year, or 365.25
Word Origins days. Neptune, the most distant planet, takes 165 years, or
more than 60,225 days. The relative diameters of the planets
Mars was named for the Roman god are shown in Figure 6. The diameters of large planets, such as
of war. Its moons, Phobos and Jupiter and Saturn, are still only a fraction of the sun’s diameter.
Deimos, were named for Mars’s sons.
Look up the names of moons of other Satellites orbit objects that have a larger mass.
planets, and see how they relate to A satellite is an object in orbit around a body that has a
the name of the planet they orbit.
larger mass. The moon is Earth’s satellite because Earth has
the larger mass. The four planets closest to the sun are small
and rocky and have few or no satellites. The next four planets
are large and gaseous and have many satellites.

All of the planets in our solar system except Mercury and
Venus have moons. Currently, we know of more than 135
natural satellites, or moons, orbiting the planets in our solar
system. In 1970, we knew of only 33. Space missions have
discovered many small satellites, and more could be found in
the future. The smallest satellites are less than 3 km in diam-
eter. The largest moons in the solar system, including Jupiter’s
Ganymede and Saturn’s Titan, are larger than the planet
Mercury. All moons are held in their orbits by the gravitational
forces of their planets. Like planets, satellites can be seen
because of the sunlight they reflect.

What force keeps a satellite in orbit around

a planet?

656 C h a p t e r 1 9 The Solar System

The Moon Figure 7 The moon has dark maria
and light highlands and craters.
The moon does not orbit the sun directly; it orbits Earth at
a distance of about 384,000 km. The moon’s surface is covered satellite (SAT’l IET) a natural or artificial
with craters, mostly caused by collisions with smaller bodies body that revolves around a celestial body
early in the history of the solar system. The maria, or large, that is greater in mass
dark patches on the moon, shown in Figure 7, are areas of lava phase (FAYZ) the change in the
that flowed out of the moon’s interior, filled the impact craters, illuminated area of one celestial body as
and cooled to solid rock. V Like the sun, the moon affects life seen from another celestial body; phases
on Earth through its movements and gravitational influence. of the moon are caused by the positions of
Earth, the sun, and the moon
The moon has phases because it revolves around Earth.
The moon appears to have different shapes throughout the

month that are called phases. The relative positions of Earth,
the moon, and the sun determine the phases of the moon, as
shown in Figure 8. At any given time, the sun illuminates half
the moon’s surface, just as at any given time, it is day on one
half of Earth and night on the other half. As the moon revolves
around Earth, the illuminated portion of the moon that faces
Earth changes. When the moon is full, the half that is lit is
facing you. When the moon is new, the side that is dark is
facing you, so you cannot see it. The time from one full moon
to the next is 29.5 days, or one calendar month. In fact, many
calendars have been based on the movement of the moon.

First quarter Figure 8 Although the sun
always lights half of the moon
Waxing gibbous Waxing crescent (red circle), the moon’s
appearance from Earth (boxes)
changes over time.

Keyword: HK8SYSF8

Full moon New moon

Waning gibbous Waning crescent

Third quarter www.scilinks.org
Topic: Moons of

Other Planets
Code: HK80993

Section 1 S u n , E a r t h , a n d M o o n 657

Modeling Eclipses 10 min

Procedure 4 Repeat step 3, but reverse the posi-
tions of the two balls. You may
1 Make two balls from modeling need to raise the small ball slightly
clay, one about 4 cm in diameter so that its shadow is centered on
and one about 1 cm in diameter. the larger ball.

2 Using a metric ruler, position the Analysis
balls about 15 cm apart on a sheet
of paper. 1. Which planetary bodies do the larger
clay ball, the small clay ball, and the
3 Turn off nearby lights. Place a pen- penlight represent?
light about 15 cm in front of and
almost level with the larger ball. 2. As viewed from Earth, what event
Shine the light on the larger ball. did your model in step 3 represent?
Sketch your model, and note the What event did step 4 represent?
effect of the beam of light.

eclipse (i KLIPS) an event in which the shadow Eclipses are caused by bodies casting shadows.
of one celestial body falls on another When Earth, the sun, and the moon are in a straight line,

Figure 9 Depending on the alignment observers on Earth may witness an event known as an eclipse.
of Earth, the sun, and the moon, an An eclipse occurs when one object moves into the shadow
eclipse may be called solar or lunar. cast by another object.
The distance between Earth and the
moon is not shown to scale. During a new moon, the moon may cast a shadow onto
Earth. Observers within that small shadow on Earth see the
sky turn dark as the moon blocks out the sun. This event is
called a solar eclipse. When the moon is full, it may pass into
the shadow of Earth. All the observers on the nightside of
Earth can see the full moon darken as the moon passes
through Earth’s shadow. This event is called a lunar eclipse.
Both of these eclipses are shown in Figure 9.

What causes a lunar eclipse?

Solar eclipse

Sun·s rays

Umbra

Penumbra

Lunar eclipse Umbra Sun·s rays

Penumbra

658 CChhaapptteerr 1X9 C h a Tpht ee r SToi ltal er S y s t e m

Why It Matters

A Return to the Moon

NASA’s Apollo program first landed a man on the moon in 1969, but
the program was shut down in 1972. No human has been back to the
moon since. However, after nearly 40 years, NASA has turned its
attention to the idea of another moon landing. New spacecraft are
currently being planned with the goal of landing humans on the moon
again by 2020.

1 NASA’s current plan is to build a 2 Once on the moon, astronauts will

new Crew Exploration Vehicle (CEV) look for resources and building

that will land our astronauts safely sites for more permanent moon

on the moon. bases. The landing vehicle itself

will carry the living quarters.

3 Returning to the moon will make www.scilinks.org
Topic: History of
it easier to get to Mars. We can
NASA
test life-support technologies and Code: HK80745

launch spacecraft from the moon. UNDERSTANDING CONCEPTS
1. Why might we want to use the

moon as a launch pad for
landing humans on Mars?
CRITICAL THINKING
2. What are some possible
benefits of humans returning
to the moon?

659

Orbit of Moon
moon

Pull of moon The moon affects Earth’s tides.
Most coastal areas on Earth, such as the one shown in
Low tide
Figure 10, have two high tides and two low tides each day. Even
High tide though tides are affected by Earth’s landscape, tides are main-
ly a result of the gravitational influence of the moon. The
Average moon’s gravitational pull is strongest on the side of Earth near-
sea level est the moon. On this side, the ocean is pulled toward the
moon, so a slight bulge is created. The solid Earth also moves
Earth slightly under the moon’s gravity, but the movement of water
is more noticeable because water is a liquid.
Figure 10 The gravitational pull of
the moon is the main cause of tides on Because Earth rotates, one area on Earth will have two
Earth. maximum, or high, tides and two minimum, or low, tides in a
period of 24 h 50 min. Because the moon is also orbiting Earth,
the times of these tides change throughout the month.

The sun also has an effect on tides, although it is minor.
When the sun is on the same side of Earth as the moon, the
gravitational forces are at their strongest and tides are at their
highest for the month.

Section 1 Review

KEY IDEAS 8. State the moon’s role in the formation of tides.

1. Explain why objects in the sky appear to move. CRITICAL THINKING

2. Describe the star that is closest to Earth. 9. Making Inferences How might the patterns of
plants and animals change if the sun were not vis-
3. Describe the basic structure of the solar system. ible to them?

4. Explain how gravity keeps planets in orbit around 10. Analyzing Conclusions The Greeks thought that
the sun. there were five planets visible with the unaided
eye: Mercury, Venus, Mars, Jupiter, and Saturn. What
5. Predict which satellite experiences the larger gravi- other planet is visible with the unaided eye?
tational force if two satellites have the same mass
but one is twice as far away from the planet. 11. Applying Concepts At what phase of the moon
will tides be the highest? Explain.
6. Describe two features of the moon, and explain
how they formed.

7. Explain what happens during a lunar eclipse and
what phase the moon is in during a lunar eclipse.

660 C h a p t e r 1 9 The Solar System

2 The Inner and
Outer Planets

Key Ideas Key Terms Why It Matters
V How are the inner planets similar to one another?
V What are gas giants? terrestrial planet Since 1959, scientists
V What type of bodies lie beyond the gas giants? hydrosphere from many countries
asteroid have launched probes
dwarf planet to study the planets
gas giant and other objects in
our solar system.

The solar system has inner planets close to the sun and

more-distant outer planets. It also has several regions full of

debris in the form of asteroids and other small bodies. The

inner planets receive more of the sun’s energy and have higher

temperatures than the outer planets do.

The Inner Planets terrestrial planet (tuh RES tree uhl
PLAN it) one of the highly dense planets
The orbits of the terrestrial planets—Mercury, Venus, nearest to the sun; Mercury, Venus, Earth,
Earth, and Mars—are shown in Figure 1. V These terrestrial and Mars
planets are relatively small and have solid, rocky surfaces.
Using telescopes, satellites, and surface probes, scientists can
study the geologic features of these planets.

The inner planets have similar compositions and share
many similar surface features. The inner planets have metallic
cores and rocky surfaces with some of the same terrain fea-
tures as Earth, including mountains, canyons, and craters.

.BST

Figure 1 Notice that &BSUI
gravity keeps the orbits 7FOVT
of the four terrestrial
planets in nearly circular .FSDVSZ
paths around the sun.

4VO

661

Mercury Mercury has extreme temperatures.
Until we sent space probes, such as Mariner 10, to investi-
Diameter 38% of Earth’s
gate Mercury, we did not know much about it. The photo-
Density 98% of Earth’s graph in Figure 2 shows that Mercury, much like Earth’s moon,
is covered with craters. Because Mercury has such a small
Surface gravity 38% of Earth’s orbit around the sun, it is never very far from the sun.

Crust Distances in the solar system are often measured in terms
Mantle of the distance from Earth to the sun, which is one astronomi-
cal unit (AU), or 150 million kilometers. Mercury is 0.4 AU
Core from the sun. Mercury’s surface temperature can reach more
than 720 K. The temperature on Mercury’s nightside drops to
Figure 2 Mercury, the planet closest 103 K, which is far below the freezing point of water. A day on
to the sun, is pocked with craters. Mercury, or the time it takes to rotate once on its axis, lasts
nearly 59 Earth days. But a year on Mercury, the time it takes
to orbit the sun, is only 88 Earth days long, or 0.24 Earth years.
Mercury has almost no atmosphere and no water.

Thick clouds on Venus cause a greenhouse effect.
Venus, shown in Figure 3, is 0.7 AU from the sun. It can be

seen near sunrise or sunset and is called the morning or
evening star. The surface of Venus has numerous mountains
and plains. Venus spins very slowly in a direction opposite
that of most other planets and the sun. One day on Venus is
243 Earth days long, and a year on Venus is 0.6 Earth years
long, or 225 Earth days. So, Venus’s day is longer than its year!

Venus does not provide an environment that can support
life as we know it. Venus is hot, and its atmosphere contains
large amounts of sulfuric acid. In addition, the atmospheric
pressure at the surface is more than 90 times the pressure on
Earth. Venus’s thick atmosphere, made up mostly of carbon
dioxide, absorbs the radiation emitted by the sun-warmed
planet. The result is a “runaway” greenhouse effect that raises
atmosphere temperatures and keeps the surface temperature

greater than 700 K. A greenhouse effect occurs when
infrared radiation is absorbed and heat builds up.

Figure 3 The Magellan spacecraft Crust
used radar to map the surface below
Venus’s thick clouds.

Venus Mantle
Core
Diameter 95% of Earth’s

Density 89% of Earth’s

Surface gravity 91% of Earth’s

662 CChhaapptteerr 1X9 C h a Tpht ee r SToi ltal er S y s t e m

Earth has ideal conditions for living creatures. Academic Vocabulary
Earth, our home, is the third planet from the sun. We
sustain (suh STAYN) to support
measure other planets in the solar system in relation to Earth.
Earth rotates on its axis in 1 Earth day. It revolves around the hydrosphere (HIE droh SFIR) the
sun at a distance of 1 AU in 1 Earth year. It has a mass of portion of Earth that is water
1 Earth mass.

Earth is the only planet we know that sustains life. It is also
the only planet that has large amounts of liquid water on its
surface. All the water on Earth’s surface, both liquid and
frozen, is called the hydrosphere. Because water takes a
relatively long time to heat or cool, the hydrosphere helps
moderate the temperature of Earth.

Name two ways that Earth is different from

the other planets.

The atmosphere protects Earth from radiation.
Earth, shown in Figure 4, has an atmosphere composed of

78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, and
other gases. The atmosphere helps moderate temperatures
between day and night. Because of the greenhouse effect, the
atmosphere absorbs energy radiated by the sun, so Earth’s
surface does not freeze at night.

Earth’s atmosphere protects us from some harmful ultra-
violet radiation and high-energy particles from the sun. The
radiation and particles are blocked in our upper atmosphere
before they can cause damage to life on Earth. The atmos-
phere also protects us from space debris, formed of leftover
portions of artificial satellites or small rocks from space. As
they speed through the atmosphere toward Earth’s surface,
these objects heat up and vaporize or shatter. Only very large
objects can survive the trip through Earth’s atmosphere.

Earth’s original atmosphere changed over time as
gases were released from microbes and, later, by plants
during photosynthesis. Earth’s early atmosphere con-
tained more carbon dioxide than it does now, as well as
other gases such as methane and ammonia.

Asthenosphere Figure 4 An image of Earth
Lithosphere taken from space shows
clouds, oceans, and land.

Mesosphere Earth

Outer Diameter 12,756 km
core
Inner core Density 5.515 g/cm3

Surface gravity 9.8 m/s2

Section 2 T h e I n n e r a n d O u t e r P l a n e t s 663

Figure 5 This artist’s rendition shows Many missions have explored the planet Mars.
a Mars Exploration Rover. Two of these Although humans have yet to visit Mars, many probes have
robots, named Spirit and Opportunity,
landed on Mars in January 2004. landed on its surface. Viking 1 and Viking 2 each sent a lander
to the surface in 1976. In 2004, two Mars Exploration Rovers,
similar to the one shown in Figure 5, landed on Mars.

White regions at the poles of Mars can be seen in Figure 6.
These are polar icecaps made of frozen carbon dioxide and
may also contain small amounts of frozen water. Features on
other parts of the planet suggest that water once flowed across
the surface as a liquid. Mars has a very thin atmosphere,
composed mostly of carbon dioxide. Mars is 1.5 AU from the
sun and has two small satellites, Phobos and Deimos. The
mass of Mars is 11% of Earth’s mass. Mars orbits the sun in 1.9
Earth years, and its day is 24.7 Earth hours. Mars is very cold;
its surface temperature ranges from 144 K to 300 K.

Mars has many extreme landforms.
Orbiting space missions have detected some unique

features on Mars. The Martian volcano Olympus Mons is the
largest mountain in the solar system. It is almost 3 times the
height of Mount Everest. Because Mars has no plate tectonics,
the volcanic lava flows remain in the same location, and
volcanoes can grow very large.

Like Earth’s moon, Mars has many impact craters. Its thin
atmosphere does not burn up objects from space, so they
often impact the surface. Also, the lack of liquid water slows
down the weathering of these craters.

The surface of Mars is red from iron oxide in its soil. Mars
has frequent dust storms that are stronger than those in the
Sahara. These dust storms form large, red dunes.

What compound causes the surface of Mars

to appear red?

Figure 6 The Hubble Space Crust
Telescope captured this striking
image of Mars.

Diameter Mars Mantle
Density 53% of Earth’s Core
Surface gravity 71% of Earth’s
38% of Earth’s

664 C h a p t e r 1 9 The Solar System

Why It Matters Phoenix The Mars lander
Phoenix was designed to
Exploring Planets drill into the Martian soil
and search for water.
Because we cannot send people to other planets yet,
scientists use robots instead. These robots are explorers, 2007
geologists, journalists, and ambassadors that venture into
the unknown. They record what they find so that we can
learn from it. Scientists have been launching spacecraft for
only about 50 years, but we have already learned a lot of
new information from the data they have collected and sent
back to Earth.

Selected planetary
missions

Luna The Russian
probe Luna 2 was the
first probe to reach the
lunar surface, making
the trip from Earth in
about 33.5 hours.

1959

Pioneer Pioneer 10
explored Jupiter and
became the first
human-made craft to
venture into the
Kuiper-Belt region.

1972

1977 Voyager The probe www.scilinks.org
1997 Voyager 2 explored Topic: Space Probes
Jupiter, Saturn, Uranus, Code: HK81432
and Neptune, along
with many of their CRITICAL THINKING
moons. 1. Why might scientists want to

Cassini-Huygens send robotic probes instead of
This probe team was humans to planets such as
designed to study Venus and Saturn?
Saturn. The Huygens
probe landed on the 665
moon Titan. Cassini
continued to orbit
Saturn and its moons.

Figure 7 At 58 km long, Ida An asteroid belt lies beyond the orbit of Mars.
is large enough to have Between Mars and Jupiter lie hundreds of smaller, rocky
captured its own small
satellite, named Dactyl. objects that range in diameter from 3 km to 700 km. These
objects are called asteroids, or small solar system bodies. The
Academic Vocabulary asteroid Ida, photographed in 1993, is shown in Figure 7. Most
asteroids remain in orbit between Mars and Jupiter, but some
attribute (uh TRIB yoot) to indicate as the wander away from this region and may cross Earth’s orbit. The
cause of odds of a large asteroid hitting Earth are very small, but many
research programs are tracking asteroids. Mass extinctions on
Earth have been attributed to past collisions with an asteroid.

The largest celestial body in the asteroid belt is Ceres,
which has a diameter of 940 km. Initially, many scientists
thought that Ceres was a planet because of its size and loca-
tion. It was later classified as an asteroid. In addition to being
an asteroid, Ceres is also considered to be a dwarf planet, a
celestial body that orbits the sun, is round because of its own
gravity, but has not cleared its orbital path.

asteroid (AS tuhr OYD) a small, rocky The Gas Giants
object that orbits the sun; most asteroids
are located in a band between the orbits of The orbits of the planets most distant from the sun—
Mars and Jupiter Jupiter, Saturn, Uranus, and Neptune—are shown in Figure 8.
V The outer planets are much larger than the inner planets
dwarf planet (DWAWRF PLAN it) a and have thick, gaseous atmospheres, many satellites, and
celestial body that orbits the sun, is round rings. These large planets are called the gas giants.
because of its own gravity, but has not
cleared its orbital path Because the gas giants have no solid surface, a spacecraft
cannot land on them. However, the Pioneer missions,
gas giant (GAS JIE uhnt) a planet that launched in 1972 and 1973; the Voyager 1 and Voyager 2
has a deep, massive atmosphere, such as missions, launched in 1977; and the Galileo mission, launched
Jupiter, Saturn, Uranus, or Neptune in 1989, investigated the large outer planets. The Cassini-
Huygens probe reached Saturn in 2004.

Name three ways in which the gas giants
differ from the inner planets.

/FQUVOF

6SBOVT

4BUVSO
+VQJUFS

Figure 8 Jupiter, Saturn,
Uranus, and Neptune—the
gas giants—are the most
distant planets known in the
solar system.

666 C h a p t e r 1 9 The Solar System

Figure 9 The storm Transition Liquid metallic
known as the Great zone hydrogen
Red Spot can be seen
in the lower portion Fluid hydrogen
of this photograph of and helium
Jupiter.
Cloud
layer

Rocky iron
core?

Jupiter

Diameter 11 times Earth’s

Density 24% of Earth’s

Surface gravity 2.54 times Earth’s

All the gas giants have rings and satellites. Mnemonic
Although the vast rings of Saturn were recognized in 1659,
Create a mnemonic to remember
it took modern technology to discover the thin, faint rings the moons of Jupiter that Galileo
around the other gas giants. Uranus’s rings were not dis- discovered. They are listed in the
covered until 1977. Most of the known satellites in the solar text from largest to smallest.
system were discovered through space missions. Jupiter has
more than 60, Saturn has more than 40, Uranus has at least 27, www.scilinks.org
and Neptune has at least 13. Most are cratered, and some have Topic: Planets
thin atmospheres. Code: HK81152

Jupiter is the largest planet in the solar system.
Jupiter, shown in Figure 9, is the first planet beyond the

asteroid belt and the largest planet that orbits the sun. Jupiter
is 1,300 times the size of Earth. If it were 80 times as massive as
it is, it could have become a star. At a distance of 5 AU, Jupiter
takes about 12 Earth years to orbit the sun; however, a day on
Jupiter is less than 10 Earth hours long. Images of Jupiter’s
atmosphere show swirling clouds of hydrogen, helium, meth-
ane, and ammonia. Complex features in Jupiter’s atmosphere
appear to be jet streams and enormous storms. One of these
storms, the Great Red Spot, is a huge hurricane that measures
more than twice the diameter of Earth. The Great Red Spot has
existed for hundreds of years.

In 1610, Galileo discovered Jupiter’s four largest satellites,
which he named Ganymede, Callisto, Io, and Europa. Using
binoculars or a small telescope, you can see them near Jupiter.
Io has a thin atmosphere and active volcanoes. Europa may
have liquid water under its icy surface.

Section 2 T h e I n n e r a n d O u t e r P l a n e t s 667

20 min Saturn has the most extensive ring system.
Saturn is 95 times the mass of Earth and takes more than
Planetary Distances
29 years to orbit the sun. However, despite its large size, a day
1 Using a large open space, such as a on Saturn is only 10.7 h long. Like Jupiter, it is a gas giant and
football field, have one classmate rotates fastest at the equator and slower near the poles. In
stand at one end of the space. This addition to its many satellites, Saturn has a spectacular system
person represents the sun. of rings, as shown in Figure 10.

2 Have another person, representing These rings are narrow bands of tiny particles of dust, rock,
Earth, stand 1 m away from the first and ice. The particles range in size from a few millimeters to
person. several meters. Most are probably the size of a large snowball
on Earth. Competing gravitational forces from Saturn and its
3 Other classmates should calculate many satellites hold the particles in place around the planet.
where they would stand if they were The rings are rather thin in comparison to their diameter.
the other planets. Many are only 10 or 20 m thick and stretch around the entire
planet. Scientists are not sure exactly how the rings formed.
4 Research other bodies in the solar Some think that they came from a smashed satellite. Others
system, such as Kuiper Belt objects. think that the rings formed from leftover material when Saturn
Have classmates stand where these and its satellites formed long ago.
objects would be. What is the far-
thest distance that two people stood Saturn may still be forming.
from each other? What objects in Saturn radiates 3 times as much energy as it receives from
the solar system did they represent?
the sun. Scientists think that helium in Saturn’s outer layers is
condensing and falling inward. As the helium nears the cen-
tral core, the gas heats up. Think about pumping air into a
bicycle tire. As you pump, the air inside the tire compresses,
which causes the tire to heat up. Eventually, for both the tire
and Saturn, the extra energy is radiated away. When Saturn
uses up its atmospheric helium, this process will stop and
Saturn will reach a state of equilibrium. Until then, Saturn is
considered to still be forming.

Figure 10 You can see Saturn
the shadow of the rings
on Saturn’s surface if Diameter 9.4 times Earth’s
you look closely.
Density 13% of Earth’s

Surface gravity 1.07 times Earth’s

Liquid
hydrogen

Liquid metallic
hydrogen

Icy core

Rocky iron
core?

668 C h a p t e r 1 9 The Solar System

Uranus and Neptune are blue gas giants. Uranus
Beyond Saturn lie the planets Uranus and Neptune, which
Diameter 4 times Earth’s
are shown in Figure 11 and Figure 12, respectively. These two
gas giants are similar to each other in size and color. Although Density 24% of Earth’s
they are smaller than Saturn and Jupiter, they are still large
enough to hold thick, gaseous atmospheres composed of Surface gravity 91% of Earth’s
hydrogen, helium, and methane. The methane gives both
planets a bluish color. 'MVJE
IZESPHFO
William Herschel discovered Uranus in 1781. He wanted to
name it after King George III, but another astronomer sug- *DZ MJRVJE
gested that it be given a name from mythology, as had the NBOUMF
other planets. Uranus is about 14 Earth masses, and it takes
approximately 84 years to orbit the sun at its distance of 19 AU. 3PDLZ JSPO
DPSF
After Uranus was discovered, astronomers used what they
knew about gravity to guide their search for other planets. Figure 11 Methane in the atmosphere
Because every mass attracts every other mass, changes in the of Uranus gives the planet a blue color.
expected orbit of Uranus could be used to predict the exist-
ence and position of other planets. Because of these changes
in Uranus’s orbit, Neptune was discovered in 1846 by Johann
Galle. It is 17 Earth masses and takes approximately 165 years
to orbit the sun at a distance of 30 AU.

The gas in the upper atmospheres of Uranus and Neptune
is very cold, about 58 K. A day on Uranus is about 17.25 h, but
its pole is tilted on its side at a 98° angle. Because of this tilt,
Uranus has the most extreme seasons in the solar system. The
few clouds in the atmosphere of Uranus show wind speeds of
200 to 700 km/h. A day on Neptune is 16 h. Neptune also has
storm systems similar to Jupiter’s. Winds on Neptune may
reach speeds of 1,100 km/h.

How did astronomers predict the existence
of the planet Neptune before discovering it?

Fluid Figure 12 Neptune is
hydrogen similar in composition and
appearance to Uranus.

Icy liquid Neptune
mantle
Diameter 3.9 times Earth’s
Rocky iron
core? Density 32% of Earth’s

Surface gravity 1.2 times Earth’s

Section 2 T h e I n n e r a n d O u t e r P l a n e t s 669

Pluto Nix Beyond the Gas Giants
Charon Hydra
V Beyond the gas giants are numerous small bodies
Figure 13 Pluto’s main satellite, composed of ice and rock. While many of these bodies are
Charon, has a diameter almost half relatively small, some are rather large. One of these larger
Pluto’s diameter. Pluto was discovered bodies, Pluto, was even considered to be a planet until 2006.
in 1930, and Charon was discovered in
1978. Two satellites, Hydra and Nix, Not all large objects in the solar system are planets.
were discovered in 2005. Until 2006, the word planet never had a clear scientific

definition. But as technology allowed new bodies beyond
Neptune to be discovered, astronomers knew that a clear
definition of planet was needed. The International
Astronomical Union (IAU) met in 2006 and voted on a defini-
tion for planet. Because Pluto has not cleared the area around
its orbital path of debris, it does not meet the full requirements
of the definition. It was reclassified as a dwarf planet.

Pluto, shown in Figure 13 with its satellites, has only a thin,
gaseous atmosphere and a solid, icy surface. It orbits the sun
in a long ellipse. It is half as large as Mercury, and its mass is
only 0.002 Earth’s mass. Pluto’s average distance from the sun
is almost 40 AU. It takes 248 Earth years to complete one orbit.

There are many objects beyond Neptune.
Beyond Neptune lies the Kuiper Belt (KIE puhr BELT). This

region is populated by many small bodies made of ice and
rock, including Pluto. Scientists think that these bodies are the
remnants of the material that formed the early solar system. In
recent years, astronomers have found several large bodies in
this region. One of these, named Eris, is larger than Pluto and
is also considered a dwarf planet. It orbits the sun at distances
reaching more than 90 AU. Astronomers are actively exploring
this region of space.

Section 2 Review

KEY IDEAS CRITICAL THINKING
1. List three ways in which the four terrestrial planets
7. Applying Concepts Why are space missions and
are similar to one another. robotic probes necessary for scientists to learn
about other planets?
2. Identify the only terrestrial planet that supports life.
8. Making Inferences Why might the gas giants
3. Explain why the surface of Venus is so hot. have so many more moons than the terrestrial
planets do?
4. Describe the features that distinguish a gas giant
from a terrestrial planet. 9. Evaluating Ideas Why might it have been impor-
tant for scientists to develop a clear definition for
5. Explain why Saturn is thought to still be forming. the word planet?

6. Describe the types of objects that are found in the
Kuiper Belt.

670 C h a p t e r 1 9 The Solar System

3 Formation of the
Solar System

Key Ideas Key Terms Why It Matters

V How did early astronomers understand and describe nebular By understanding how
hypothesis our own solar system
the solar system? formed, astronomers
nebula will know what to look
V Why is the solar system organized like it is? comet for around other stars
V What else is in the solar system besides planets? exoplanet when they look for new
V How did Earth’s moon form? planets.
V How do astronomers find planets around other stars?

The oldest record of human interest in astronomy was found

in Nabta, Egypt. Scientists and historians think that a group of

stones were specially arranged 6,000 to 7,000 years ago to line

up with the sun on the summer solstice.

Early Astronomy Figure 1 Stonehenge, located in
England, is one of the world’s oldest
Many ancient peoples watched for changes in the sky. observatories.
Stonehenge, shown in Figure 1, was probably used for keeping
time. Its stones are aligned with the seasonal solstices.

Ancient peoples used myths to explain star movements
that they did not understand. V Eventually, mathematical
tools began to be used to make more accurate models of
observed astronomical objects. Ancient questions about
how the universe worked and how it was organized helped
develop science and the scientific method.

671

10 min The first model put Earth at the center.
Like many people who came before them, the ancient
Estimating 4.6
Billion Greeks observed the sky to keep track of time. But they took a
new approach in trying to understand Earth’s place in the
1 Find a small box, and measure its universe. They used logic and mathematics, especially geom-
length, width, and height etry. The Greek philosopher Aristotle explained the phases of
the moon and eclipses by using a model of the solar system
2 To find the volume of the box, multi- with Earth at the center. His geocentric, or “Earth-centered,”
ply the height by the width by the model is shown in Figure 2.
length.
This model was expanded by Ptolemy in 140 ce. Ptolemy
3 Fill the box halfway with popcorn thought that the sun, moon, and planets orbited Earth in
kernels. perfect circles. His theory described what we see in day-to-day
life, including motions of the sun and planets. Because it
4 Count the number of kernels in the predicted many astronomical events well, Ptolemy’s model
box, and multiply that number by 2. was used for more than a thousand years.

5 Divide 4,600,000,000 by the total Copernicus’s model put the sun at the center.
number from step 4. In 1543, Nicolaus Copernicus proposed a heliocentric, or

6 How big would your box need to be “sun-centered,” model. He realized that many adjustments
to hold 4.6 billion popcorn kernels? used to make Ptolemy’s model work would not be needed in a
model in which the sun was at the center. In this new model,
7 If you have time, estimate the Earth and the other planets orbit the sun in perfect circles.
weight of 4.6 billion kernels. Although Copernicus’s model was not perfect, it explained the
motion of the planets more simply than Ptolemy’s model did.
Figure 2 Before scientists were able to In 1605, Johannes Kepler further improved the model by
prove that Earth revolved around the proposing that the orbits around the sun are ellipses, or ovals,
sun, most people believed that Earth rather than circles.
was in the center of the universe.
Name the main way in which the
Medieval art of the solar system heliocentric model differs from the geocentric model?

Geocentric model Heliocentric model

Epicycle Moon
Earth Earth

Sun

Sun

Sphere of stars

672 C h a p t e r 1 9 The Solar System

Newton explained it all. Chain-of-Events Chart
No one could explain why planets orbited the sun in
Create a chain-of-events chart to
elliptical orbits until 1687. Isaac Newton was the first to ex- describe the formation of the solar
plain that gravity keeps the planets in orbit around the sun system and the concept of the
and satellites in orbit around planets. His theory states that nebular model.
every object in the universe exerts a gravitational force on
every other object. Newton was the first to propose that every-
thing in the universe follows the same rules and acts in a
predictable way. All of classical physics, including much of
astronomy, is built on this assumption.

The Nebular Hypothesis Academic Vocabulary

From their dating of rocks, scientists estimate that the solar approximate (uh PRAHK suh mit) almost;
system is approximately 4.6 billion years old. Scientists trying about
to build a good model to explain how the solar system formed
started by asking questions: Why are the planets almost in the nebular hypothesis (NEB yu luhr
same plane? Why are their orbits nearly circular? A good hie PAHTH uh sis) a model for the
model would also have to explain the presence and behavior formation of the solar system in which
of objects such as satellites, comets, and asteroids. the sun and planets condense from a cloud
(or nebula) of gas and dust
Scientists developed the nebular hypothesis, which is nebula (NEB yu luh) a large cloud of
shown in Figure 3 on the next page, to explain how our solar dust and gas in interstellar space; a region
system formed. A nebula is a large cloud of dust and gas in in space where stars are born
space. V This hypothesis explains why objects that form
from a disk will lie in the same plane, have almost circular
orbits, and orbit in the same direction.

Planets formed by the accretion of matter in the disk.
The nebular hypothesis states that planets formed mostly

through the process by which small particles collide and stick
together, called accretion. The nebular hypothesis also ex-
plains why the terrestrial planets are different in composition
from the gas giants. Warm temperatures near the sun pre-
vented light gases from remaining in the atmospheres of the
terrestrial planets. Colder gas and dust in the outer part of the
disk became the gas giants. These planets were large enough
and cold enough to hold light nebular gases, such as hydro-
gen, in their atmospheres.

The nebular hypothesis explains smaller rocks in space.
Satellites may have formed around gas giants through the

process of accretion. Another possibility is that particles that
might have formed planets were captured by the gravitational
pull of the gas giants. Some smaller satellites may have broken
off from larger ones. Most satellites orbit planets in the same
direction that the planets orbit the sun.

Section 3 F o r m a tSi oenc t oi of nt hXe SSeocl tairo nS yTsittel me 673

Figure 3 The Nebular Hypothesis

1 2 3

According to the nebular As the cloud collapsed, it formed The spinning motion of the disk
hypothesis, the sun, like every into a rotating disk. In the center, caused it to flatten. Planetesimals,
star, formed from a cloud of gas where the material became or particles that become planets,
and dust that collapsed because denser and hotter, a star began to began to form in the disk. Their
of gravity. In our solar system, form. As the cloud continued to formation caused more changes
this process began almost collapse, it spun faster and faster. in the disk.
5 billion years ago.

4 5 6

As the planetesimals grew, their Small planetesimals collided with Because each planet swept up the
gravitational pull increased. The larger ones, and the planets material in its region, the planetary
largest planetesimals began to began to grow larger and more orbits are separate from each
collect more of the gas and dust stable. The warmer, inner planets other. Asteroids and other small
of the nebula. were rocky, and the colder, outer rocks are most likely leftover debris
planets accumulated lightweight from solar system formation.
gases in their atmospheres.

674 C h a p t e r 1 9 The Solar System

Rocks in Space

V There are many types of small bodies in our solar
system, including satellites, comets, asteroids, and meteor-
oids. Satellites orbit planets, and comets are probably com-
posed of material left over from the formation of the solar
system. Most asteroids in our solar system can be found
between Mars and Jupiter. Meteoroids are small pieces of rock
that enter Earth’s atmosphere. Most meteoroids that strike
Earth burn up in the atmosphere, and we see them as meteors
streaking through the night sky. If a meteoroid does not com-
pletely burn up in the atmosphere and hits the ground, it is
called a meteorite.

Comets may give clues to the origin of the solar system. Figure 4 The many pieces of the
By studying comets, scientists have gained important comet Shoemaker-Levy 9 hit Jupiter.

information about the material that made the solar system. comet (KAHM it) a small body of ice,
Comets are composed of dust and of ice made from methane, rock, and cosmic dust that follows an
ammonia, carbon dioxide, and water. In 1994, pieces of the elliptical orbit and that gives off gas and
comet Shoemaker-Levy 9 collided with Jupiter, as shown in dust in the form of a tail as it passes close to
Figure 4. Analysis of the impacts revealed that the comet also the sun
contained silicon, magnesium, and iron. Because of their
composition, comets are sometimes called dirty snowballs.

Comets have long tails and icy centers.
When a comet passes near the sun, the comet gives off

gases in the form of a long tail. Some comets, such as the one
shown in Figure 5, have two tails—an ion tail made of charged
particles that is blown by the solar wind and a dust tail that
follows the comet’s orbit.

A comet’s orbit is usually very long. When its tail even-
tually disappears, a comet becomes more difficult to see. It
will brighten only when it passes by the sun again.

Why is it easier to see a comet when it is

near the sun?

Figure 5 In this photograph, you can
see the two tails of the comet Hale-
Bopp. The blue streak is the ion tail,
and the white streak is the dust tail.

Section 3 F o r m a t i o n o f t h e S o l a r S y s t e m 675

Where do comets come from?

Oort cloud Orbit of During the formation of our solar system,
long-period some small planetesimals did not combine
comet

with other planetesimals. These leftovers

Orbit of Inner solar strayed far from the sun and developed very
Neptune system long orbital periods. These far-flung pieces
make up the Oort cloud of comets, shown in
Kuiper Belt Figure 6. The Oort cloud may be up to

100,000 AU wide and extend in all directions.

Orbit of Planetesimals that remained in the nebular
short-period disk formed the Kuiper Belt beyond the orbit
comet of Neptune. Most of these small bodies are

rocky and covered by ice. Most comets come

from this region of the solar system.

Halley’s comet is one of the most famous

comets. It travels in a highly elliptical orbit

Figure 6 Comets come from the Oort that takes it out into the Kuiper Belt. It appears in Earth’s sky
cloud, a spherical region in the outer once every 76 years. In contrast to the other bodies in the solar
solar system beyond the Kuiper Belt, a system, Halley’s comet orbits backward. Its orbit was probably
disk-shaped region beyond the orbit of greatly altered by a planet’s gravity.
Neptune.

Describe the type of objects that make up

the Kuiper Belt.

Meteorites can be made of many elements.
We can study asteroids by studying meteorites. Many of

the larger rocks that make it through Earth’s atmosphere come
directly from asteroids. As shown in Figure 7, there are three
major types of meteorites. Stony meteorites include carbon-
rich specimens that contain organic materials and water. Iron
meteorites are made mostly of iron and nickel. Stony-iron
meteorites are a combination of the two types. Most meteor-
ites that have been collected are stony and have compositions
like those of Earth and the moon.

Figure 7 There are three major types of meteorites.

Stony Iron Stony-iron
Rocky material Iron and nickel Rocky, iron and nickel

676 CChhaapptteerr 1X9 C h a Tpht ee r SToi ltal er S y s t e m